1. Field of the Invention
The present invention generally relates to high-speed data transmission. More specifically, the present invention relates to a simple transmit equalization system that pre-distorts a data signal in order to compensate for frequency dependent amplitude variations in channel media.
2. Discussion of the Related Art
In recent years, the speed of digital data signals on printed circuit boards (PCBs) and in system backplanes between components is increasing as integrated circuit (IC) technology becomes faster and faster.
The most common method of transferring data between chips is via a parallel bus. A parallel bus includes, for example, a group of data lines and a clock, in which the clock is transmitted along with the data in order to latch in the data at the far end of the channel, or at the next chip. As clock rates reach 300 MHz and above, it becomes increasingly more difficult to implement a parallel bus due to skew between the clock and data lines, as well as transmission line effects.
Instead of using parallel buses at clock rates above 300 MHz, serial buses are utilized. In a serial bus, the parallel data is encoded, and sent without a clock. The purpose of encoding is to embed repetitive pattern transitions along with the data so that a far end device receiving the serial data can extract the clock from the data stream. The extracted clock signal is used to recover the data.
The skew between multiple serial bus lines is handled by placing digital alignment patterns inside the data so the far end device, such as an application-specific integrated circuit (ASIC), can realign the data streams with digital logic. Because the serial bus eliminates the clock line and embeds the clock information within the data stream, a serial bus is able to successfully function over longer distances than a parallel bus, if all other factors are equal.
Serial buses are extended further if equalization is used to cancel out the degrading effects of the PCB metal traces on the serial bus signal. It is known in the art to use “receive equalization” to correct signals for the effects of the PCB metal traces. “Receive equalization” is the modification of the serial signal at the receiver to compensate for the degrading effects of the PCB metal traces on the serial signal. However, at very high speeds (>1 GHz), receive equalization is difficult to implement and consumes a large area on the receiver side, as well as a large amount of power on the receiver side.
An additional approach for equalization is to pre-distort the transmit signal to compensate for the known degradation caused by the PCB traces. This technique is referred to as transmit equalization.
High speed communication over a serial bus is becoming commonplace in the industry, such as the serial bus interface 10 Gigabit Attachment Unit Interface, also known as “XAUI.” XAUI is being defined for 10 Gigabit Ethernet by the Institute of Electrical and Electronic Engineers (IEEE) 802.3ae standards body. XAUI is a 10 Gigabit serial bus using four lanes of 2.5 Gigabit data that are 8 bit/10 bit encoded into a 3.125 Gigabaud lane. XAUI is intended to be the interface that connects 10 Gigabit physical layer devices with 10 Gigabit Ethernet controller devices.
The IEEE standard requires that XAUI work over 50 cm of FR4 PCB material. Tests reveal that inter-symbol interference (ISI) is a problem after 50 cm of FR4 if no transmit equalization occurs. A superposition of possible signal traces for two symbol periods out of a long random sequence of data is referred to as an eye diagram. If ISI becomes a problem, the openings of the eye diagram become smaller. Transmit equalization increases the eye pattern opening significantly after 50 cm of PCB trace and it can extend the distance over which XAUI operates beyond 50 cm.
Most of the prior art for transmit equalization is very complex and requires a large area on silicon. One transmit equalizer performs one-bit equalization on any runlength of 0s or 1s that is greater than one. For example, a one-bit equalization scheme adjusts the input signal with the same amplitude adjustment if it encounters at least two consecutive 0s or 1s. While this equalization does compensate for a portion of the frequency response of the channel, it is not precise and does not adequately compensate for longer runs of zeros and ones.
Accordingly, there is a need for a fast transmit equalization method and system which performs at least two-bit equalization and may be implemented with simple circuitry.